An Ni-Fe alloy corresponding to PC specified in JIS (abbreviation of Japanese Industrial Standards) (hereinafter referred to as "PC permalloy") is a magnetic material widely applied for a case and a core of a magnetic head, cores of various transformers, and various magnetic sealing materials.
An ingot of the above-mentioned PC permalloy is poor in hot-workability. When the ingot of PC permalloy is slabbed, therefore, many surface flaws are produced on the resultant slab for reasons as described later.
Hot-workability of the ingot of PC permalloy varies depending upon the nickel content in the ingot. More specifically, a higher nickel content in the ingot of PC permalloy leads to a lower hot-workability of the ingot. As a result, an ingot of PC permalloy containing nickel in an amount of about 80 wt. % is far inferior in hot-workability to an ingot of an Ni-Fe alloy containing nickel in an amount of about 35 to 45 wt. %. When manufacturing a slab having a few surface flaws such as edge cracks, i.e., having an excellent surface quality, from an ingot of PC permalloy, therefore, the slabbing process could not be adopted, so that it was inevitable to adopt the forging process. The reasons are as follows: A multi-axial stress and a shearing stress mainly act on the ingot in the slabbing process, whereas a compression stress mainly acts on the ingot in the forging process. However, the forging process has a lower hot-working efficiency than in the slabbing process, and the production of surface flaws on the slab cannot largely be reduced even by adopting the forging process. It is therefore necessary to remove surface flaws on the slab even in the forging process, and this requires additional time and labor for manufacturing a slab.
When a slab is manufactured by slabbing an ingot in general or when a strip is manufactured by hot-rolling the thus prepared slab, not limited to an ingot of PC permalloy, having a poor hot-workability, many surface flaws such as edge cracks are produced on the thus manufactured slab or strip. The reason is as follows: When the ingot is slabbed or when the slab is hot-rolled, the ingot or the slab deforms at a strain rate of at least 1S.sup.-1. The edge portion and the surface layer portion of the ingot or the slab at this stage have a temperature of about 800.degree. C. lower than that at the center portion of the ingot or the slab. If the ingot or the slab having such a temperature difference is subjected to the deformation by the slabbing or the hot-rolling, therefore, surface flaws such as edge cracks are produced on the resultant slab or strip.
Particularly when the ingot of PC permalloy poor in hot-workability is subjected to the slabbing, numerous surface flaws are produced on the resultant slab. The reason is as follows: When the ingot of PC permalloy is slabbed, impurity elements segregate on the grain boundaries of austenite during the temperature decrease of the ingot, thus making the grain boundaries more brittle. As a result, ductility of the ingot is seriously deteriorated at an ingot temperature of from 800.degree. to 1,000.degree. C. This causes production of numerous surface flaws on the slab.
The above-mentioned problem is posed also when manufacturing an alloy sheet through hot-rolling of the slab or when manufacturing a press-formed article by hot-pressing the thus rolled alloy sheet.
As Ni-Fe alloys to solve these problems, the following ferromagnetic ones have been proposed.
(1) A ferromagnetic Ni-Fe alloy disclosed in Japanese Patent Publication No. 60-7,017 dated Feb. 21, 1985, which consists essentially of:
______________________________________ nickel from 75.0 to 84.9 wt. %, titanium from 0.5 to 5.0 wt. %, magnesium from 0.0010 to 0.0020 wt. %, ______________________________________
and the balance being iron and incidental impurities,
where, the respective contents of carbon and sulfur as said incidental impurities being:
up to 0.03 wt. % for carbon, and
up to 0.003 wt. % for sulfur.
(hereinafter referred to as the "prior art 1").
(2) A ferromagnetic Ni-Fe alloy disclosed in Japanese Patent Provisional Publication No. 62-227,053 dated Oct. 6, 1987, which consists essentially of:
______________________________________ nickel from 70 to 85 wt. %, manganese from 1.2 to 10.0 wt. %, molybdenum from 1.0 to 6.0 wt. %, copper from 1.0 to 6.0 wt. %, chromium from 1.0 to 5.0 wt. %, boron from 0.0020 to 0.0150 wt. %, ______________________________________
and the balance being iron and incidental impurities,
where, the respective contents of sulfur, phosphorus and carbon as said incidental impurities being:
up to 0.005 wt. % for sulfur,
up to 0.01 wt. % for phosphorus, and
up to 0.01 wt. % for carbon.
(hereinafter referred to as the "prior art 2").
(3) A ferromagnetic Ni-Fe alloy disclosed in Japanese Patent Provisional Publication No. 62-227,054 dated Oct. 6, 1987, which consists essentially of:
______________________________________ nickel from 70 to 85 wt. %, manganese up to 1.2 wt. %, molybdenum from 1.0 to 6.0 wt. %, copper from 1.0 to 6.0 wt. %, chromium from 1.0 to 5.0 wt. %, boron from 0.0020 to 0.0150 wt. %, ______________________________________
and the balance being iron and incidental impurities,
where, the respective contents of sulfur, phosphorus and carbon as said incidental impurities being:
up to 0.005 wt. % for sulfur,
up to 0.01 wt. % for phosphorus, and
up to 0.01 wt. % for carbon.
and the weight ratio of the boron content to the total content of sulfur, phosphorus and carbon as said incidental impurities being within the range of from 0.08 to 7.0.
(hereinafter referred to as the "prior art 3").
The above-mentioned prior art 1 involves the following problems: The prior art 1 is characterized in that hot-workability of the alloy is improved by fixing sulfur which is one of the impurity elements by means of magnesium which has a strong tendency to form a sulfide. However, the value of reduction of area at a temperature within the range of from 800.degree. to 1,000.degree. C., which is particularly important for the hot-working, is as low as from 40 to 60%, as disclosed in the example of the prior art 1. As a result, application of the hot-working to the alloy material of the prior art 1 causes production of many surface flaws on the obtained slab.
The above-mentioned prior arts 2 and 3 involve the following problems: The prior arts 2 and 3 are characterized in that hot-workability of the alloy is improved by reducing the contents of sulfur, phosphorus and carbon which are the impurity elements, and adding boron to inhibit segregation of the impurity elements on the grain boundaries of austenite. However, the alloys of the prior arts 2 and 3 have a very low hot-workability as described below. The alloy No. 2 disclosed in the example of the prior art 2 was melted in a vacuum melting furnace, and then cast into an ingot. Then, a test piece having a diameter of 5 mm and a length of 100 mm was cut from the thus cast ingot. The test piece was heated to a temperature of 1,200.degree. C. and then cooled to a temperature of 900.degree. C. On the thus hated and cooled test piece, a value of reduction of area was measured. The test piece showed a value of reduction of area of 20%.
The value of reduction of area is defined as follows: Assume that a tensile stress is applied in a tension test to a test piece at a strain rate of at least 1S.sup.-1 until the test piece is fractured. The value of reduction of area means a percentage ((A-A')/Ax100) of the difference (A-A') between the original sectional area (A) of the test piece and the minimum sectional area (A') thereof upon the fracture, relative to the original sectional area (A) thereof. The same applies also hereafter to the term "value of reduction of area" in all cases.
A test piece was cut from the alloy No. 5 disclosed in the example of the prior art 3 in the same manner as in the above-mentioned prior art 2, and a value of reduction of area for this test piece was measured under the same conditions as in the prior art 2. The test piece showed a value of reduction of area of 25%.
In both the prior arts 2 and 3, the value of reduction of area at 900.degree. C., which is particularly important in the hot-working, is low as described above. As a result, application of the hot-working to the alloy materials of the prior arts 2 and 3 causes production of many surface flaws on the obtained slabs.
Under such circumstances, there is a strong demand for the development of a ferromagnetic Ni-Fe alloy having an excellent hot-workability as represented by a value of reduction of area of over 60% at a temperature within the range of from 800.degree. to 1,000.degree. C. and a method for manufacturing a slab having an excellent surface quality of such an alloy, but such an alloy and a method for manufacturing such a slab of the alloy have not as yet been proposed.